A ROMP-derived, polymer-supported chiral Schrock catalyst for enantioselective ring-closing olefin metathesis

A permanently polymer-immobilised version of Schrock's molybdenum catalyst was realised via polymerisation of chiral 5,5'-bis(norborn-5-ene-2-ylmethyleneoxymethyl)-3,3'-di-tert-butyl-6,6'-dimethylbiphen-2,2'-diol followed by reaction with the catalyst precursor Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OSO2CF3)2(CH3OCH2CH2OCH3); using this 5-, 6- and 7-membered heterocycles were synthesised via ring-closing metathesis (RCM) from (pro-)chiral substrates in high yields and ee with short reaction times; the catalyst shows low loss of molybdenum during RCM, and was easily separated and recycled.     

A highly enantioselective one-pot synthesis of homoallylic alcohols via tandem asymmetric allyl transfer/olefin cross metathesis

A highly enantioselective one-pot synthesis of linear homoallylic alcohols with terminal ester functionality has been achieved. The reactions were controlled by ordered addition of reagents and catalysts, ensuring complete consumption of aldehyde. The synthetic utility of this strategy has been demonstrated in a short synthesis of a low boiling point intermediate for grahamimycin A.     

Catalytic asymmetric ring-opening metathesis/cross metathesis (AROM/CM) reactions. Mechanism and application to enantioselective synthesis of functionalized cyclopentanes

Studies regarding the first examples of catalytic asymmetric ring-opening metathesis (AROM) reactions are detailed. This enantioselective cleavage of norbornyl alkenes is followed by an intermolecular cross metathesis with a terminal olefin partner; judicious selection of olefin is required so that oligomerization and dimerization side products are avoided. Results outlined herein suggest that the presence of suitably positioned heteroatom substituents may be critical to reaction efficiency. Mo-catalyzed tandem AROM/CM affords functionalized cyclopentyl dienes in >98% ee and >98% trans olefin selectivity; both secondary and tertiary ether products can be obtained. The examples provided include the catalytic synthesis of an optically pure cyclopentyl epoxide and dimethyl acetal. Mechanistic studies suggest that it is the more substituted benzylidene or silylated alkylidenes that are involved in the catalytic process (vs the corresponding Mo-methylidenes). Although electron rich benzylidenes react more efficiently, the derived electron poor Mo complexes promote AROM/CM transformations as well; alkylidenes that bear a boron substituent are unreactive.     

Efficient enantioselective synthesis of functionalized tetrahydropyrans by Ru-catalyzed asymmetric ring-opening metathesis/cross-metathesis (AROM/CM)

An efficient method for enantioselective synthesis of highly functionalized pyrans (up to 98% ee) through Ru-catalyzed asymmetric ring-opening metathesis/cross-metathesis is described. Reactions are promoted by a recyclable chiral Ru-chloride or a new chiral Ru-iodide complex; the latter catalyst is less efficient but gives rise to significantly higher levels of enantioselectivity. Catalytic reactions can be performed in undistilled solvent and with a wide range of substrates, including those that contain secondary and tertiary alcohols. Representative regioselective functionalizations that highlight the utility of the catalytic method are also presented.     

Comparison of Ru- and Mo-based chiral olefin metathesis catalysts. Complementarity in asymmetric ring-opening/cross-metathesis reactions of oxa- and azabicycles

A comparative study of chiral Mo- and Ru-based catalysts to promote enantioselective synthesis of 2,6-disubstituted pyrans and piperidines through asymmetric ring-opening/cross-metathesis (AROM/CM) reactions is presented. These studies demonstrate the critical complementarity that exists between the two classes of chiral catalysts.     

Highly active chiral ruthenium catalysts for asymmetric ring-closing olefin metathesis

The synthesis of olefin metathesis catalysts containing chiral, monodentate N-heterocyclic carbenes and their application to asymmetric ring-closing metathesis (ARCM) are reported. These catalysts retain the high levels of reactivity found in the related achiral variants (1a and 1b). Using the parent chiral catalysts 2a and 2b and derivatives that contain steric bulk in the meta positions of the N-bound aryl rings (catalysts 3-5), five- through seven-membered rings were formed in up to 92% ee. The addition of sodium iodide to catalysts 2a-4a (to form 2b-4b in situ) caused a dramatic increase in enantioselectivity for many substrates. Catalyst 5a, which gave high enantiomeric excesses for certain substrates without the addition of NaI, could be used in loadings of < or =1 mol %. Mechanistic explanations for the large sodium iodide effect as well as possible mechanistic pathways leading to the observed products are discussed.     

Supported chiral Mo-based complexes as efficient catalysts for enantioselective olefin metathesis

Syntheses and catalytic activities of seven new polymer-supported chiral Mo-based complexes are disclosed. Four of the complexes are polystyrene-based, and three involve polynorbornene supports. Studies concerning the ability of the polymer-bound chiral complexes to promote an assortment of asymmetric ring-closing (ARCM) and ring-opening (AROM) metathesis reactions are detailed. In many instances, levels of reactivity and enantioselectivity are competitive with those of the analogous homogeneous catalysts. The positive effect of lower cross-linking within the polymer backbone on reaction efficiency and asymmetric induction is detailed. The optically enriched products obtained through the use of the supported complexes, after simple filtration and removal of the supported Mo catalysts, contain significantly lower levels of metal impurities as compared to products synthesized with the corresponding homogeneous catalysts.     

Enantioselective synthesis of cyclic allylboronates by Mo-catalyzed asymmetric ring-closing metathesis (ARCM). A one-pot protocol for net catalytic enantioselective cross metathesis

Mo-catalyzed asymmetric ring-closing metathesis (ARCM) reactions are used to synthesize cyclic allylboronates of high optical purity (89% ee to >98% ee). A one-pot procedure involving formation of allylboronates, Mo-catalyzed ARCM and functionalization of the optically enriched cyclic allylboronates constitutes net asymmetric cross metathesis (ACM). Structural modification of ARCM products include reactions with aldehydes to afford optically enriched compounds that bear quaternary carbon centers with excellent diastereoselectivity. These studies emphasize the significance of the availability of chiral Mo-based complex as a class of chiral metathesis catalysts that frequently complement one another in terms of reactivity and selectivity.     

A general model for selectivity in olefin cross metathesis

In recent years, olefin cross metathesis (CM) has emerged as a powerful and convenient synthetic technique in organic chemistry; however, as a general synthetic method, CM has been limited by the lack of predictability in product selectivity and stereoselectivity. Investigations into olefin cross metathesis with several classes of olefins, including substituted and functionalized styrenes, secondary allylic alcohols, tertiary allylic alcohols, and olefins with alpha-quaternary centers, have led to a general model useful for the prediction of product selectivity and stereoselectivity in cross metathesis. As a general ranking of olefin reactivity in CM, olefins can be categorized by their relative abilities to undergo homodimerization via cross metathesis and the susceptibility of their homodimers toward secondary metathesis reactions. When an olefin of high reactivity is reacted with an olefin of lower reactivity (sterically bulky, electron-deficient, etc.), selective cross metathesis can be achieved using feedstock stoichiometries as low as 1:1. By employing a metathesis catalyst with the appropriate activity, selective cross metathesis reactions can be achieved with a wide variety of electron-rich, electron-deficient, and sterically bulky olefins. Application of this model has allowed for the prediction and development of selective cross metathesis reactions, culminating in unprecedented three-component intermolecular cross metathesis reactions.     

An enantiomerically pure adamantylimido molybdenum alkylidene complex. An effective new catalyst for enantioselective olefin metathesis

An enantiomerically pure Mo-based complex that bears an alkylimido ligand is prepared and characterized through NMR spectroscopy and X-ray analysis. Mo complex 4 is the only reported chiral alkylimido catalyst; all previous chiral complexes are arylimido systems. These studies show that the chiral Mo catalyst exists exclusively as the syn isomer and that it offers unique reactivity and selectivity profiles in asymmetric olefin metathesis.     

A fluorous chiral dirhodium(II) complex as a recyclable asymmetric catalyst

[reaction: see text] The chiral fluorous complex tetrakis-dirhodium(II)-(S)-N-(n-perfluorooctylsulfonyl)prolinate has been prepared and used as a catalyst in homogeneous or fluorous biphasic fashion. The catalyst displays good chemo- and enantioselectivity in intermolecular cyclopropanation and C-H bond activation reactions. The catalyst can be simply and thoroughly separated from the reaction mixture and is recyclable.     

Development of a recyclable fluorous chiral phase-transfer catalyst: application to the catalytic asymmetric synthesis of alpha-amino acids

[structure: see text] A recyclable fluorous chiral phase-transfer catalyst was synthesized and successfully applied for the catalytic asymmetric synthesis of both natural and unnatural alpha-amino acids. The reaction involves alkylation of a glycine derivative followed by extractive recovery of the chiral phase-transfer catalyst using fluorous solvent.     

A new chiral diiron catalyst for enantioselective epoxidation

The dinuclear chiral complex Fe(2)O(bisPB)(4)(X)(2)(ClO(4))(4) (X = H(2)O or CH(3)CN) catalyzes with high efficiency (up to 850 TON) and moderate enantioselectivity (63%) the epoxidation of electron deficient alkenes at 0 degrees C by a peracid.     

A chiral phosphepine–olefin rhodium complex as an efficient catalyst for the asymmetric conjugate addition

A monophosphine–olefin was synthesised from a 2,2′-bridged 1,1′-binaphthyl precursor, which served as a chiral bidentate ligand in the Rh catalysed 1,4-additions of arylboronic acids to cycloalkenones and 5,6-dihydro-2H-pyran-2-one. Fair yields (64–88%) and high asymmetric inductions (88–98% ee) have been obtained. The crystal structure of a corresponding cationic Rh(I) complex was determined.     

An ionic liquid-supported ruthenium carbene complex: a robust and recyclable catalyst for ring-closing olefin metathesis in ionic liquids

The synthesis of an ionic liquid-supported olefin metathesis catalyst derived from Grubb's ruthenium carbene complex is described. This new supported catalyst has been used in BMI.PF6 solvent, and this allowed success in solving the challenging problem of catalyst recycling. The IL catalyst in BMI.PF6 can be recovered and reused up to 10 consecutive cycles in RCM reactions of several dienes with excellent conversions. Moreover, the IL catalyst shows a remarkable stability in BMI.PF6 and can be stored several months without loss of activity. These results clearly demonstrate the importance of anchoring an imidazolium ionic liquid pattern to the catalyst to avoid its leaching from the BMI.PF6 phase.